Daugherty, Carroll R.
Houghton Miffiln, 1950
Buku Teks
Commons, John R.
Macmillan, 1934
Buku Teks
Robinson, Marshall A., author
Brookings Institution, 1955
Buku Teks
Brown, A.J., author
George Allen and Unwin, 1959
Buku Teks
Brown, A.J., author
George Allen and Unwin, 1959
Buku Klasik